The present invention relates in general to methods and equipment for characterizing objects that have been made by deforming sheet material, and in particular to a new and useful computerized method of determining the surface strain distribution in such a deformed object.
A wide variety of objects are made using sheet metals and plastics that are deformed into a final shape. Examples include automobile fenders, dishwasher casings, kitchen sinks, beer cans, soft drink containers and plastic cups. During the manufacturing stage of such objects or products, it is necessary to know how the material has deformed at all locations and during every stage of the deformation. This includes the stretching, drawing or pulling of the material to form the final shape. There is a two-fold reason why this information is necessary. Firstly, this information is needed to ensure the product has uniform quality. Substantially uniform sheet thickness throughout the deformed object is desirable for example. The other reason is to assess the potential for local failure or fracture over different areas of the object.
Devices and methods are known for assessing the distribution of strain or deformation in objects that have been formed from flat sheets of material. The Balanced Engineering Corporation, markets a portable grid circle analyzer which utilizes a hand held digital camera that scans a circular grid that was placed on the surface of the sheet material before deformation. The circles which have been deformed into ellipses are mathematically analyzed to determine strain in each area on the deformed object. A similar product is available from MTS which is designated the Model 637 Optical Grid Analyzer.
Both of these devices are exceeding expensive to purchase.
Another system is described in a paper by E. Schedin and A. Melander entitled "The Evaluation of Large Strains from Industrial Sheet Metal Stampings with a Square Grid" (J. of Applied Metal Working, Vol. 4, 1986, pp. 143-156). This system is based on locating points instead of circle dimensions but uses a single view and requires a large amount of user interaction for each strain point calculation, just as required in the grid circle analyzer devices.
One system is disclosed in "Application of Stereophotogrammetic Methods to Analyze Kinematics and Dynamics of Shells" by T. Bednarski, Instytut Technologii Bezwiorowych, Politechniki Warszawskiej, 1977 (which is in Polish). This system appears to utilize two cameras to make strain measurements during the formation of an object. The details of this disclosure are not clear however.
U.S. Pat. Nos. 3,552,856 and 4,722,600 disclose methods for measuring strain using X and Y coordinate fringes to provide two or one dimensional strain analysis.
Other imaging techniques are shown in U.S. Pat. Nos. 4,115,806; 4,205,973; 4,508,452; 4,634,279; 4,645,347 and 4,657,394.
Another article which is relevant for its teaching of the use of two camera angles, is entitled "Strain Measurement of Structures with Curved Surface by Means of Personal Computer-Based Picture Processing", by A. Miyoshi, et al. (Engineering With Computers, Vol. 3, 1988, pp. 149-156).
One major drawback of all known systems for measuring strain in sheets, is that at most one strain value is obtained per measurement. According to the present invention, hundreds of strain values can be produced over a significant region of a part being tested, with just two measurements being taken.